[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT MON AUG 12, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



In the wake of an upper-level trough exiting to the north of the state, onshore flow has stabilized the air mass and forced a deep marine layer into western Oregon. Winds are light through the marine layer, with southwesterly flow above it.  With sunshine heating central Oregon, late this morning, and low clouds retarding heating west of the Cascade crest, pressure gradients have turned slightly unfavorable for burning.



Late-morning satellite imagery shows the low clouds beginning to break up over the northern Willamette Valley but still solidly covering the central and southern valley.  Skies should begin clearing over the Silverton Hills area by 2 p.m.  As the valley warms, onshore pressure gradients will become increasingly more favorable for open burning.  Low-level winds should remain fairly light and turn northwesterly.  Winds will stay southwesterly near the mixing height, which should rise to near 5000 feet by late this afternoon.  PIBALS are scheduled to begin at 1:00 p.m.  If skies clear soon enough, then the prospects are good for open burning later this afternoon.



TODAY'S FORECAST:



Morning clouds; becoming mostly sunny this afternoon.



Salem's high temperature today will be near 80 degrees (normal is 83).

Relative humidity:  Drops to near 40% by 5 p.m.

Surface winds:  Becoming NW 4-7 mph this afternoon.

Transport winds: W 6-10 mph this afternoon.

Mixing height: Rising to 4000 feet by 2 p.m. and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:22 p.m.



(Salem Airport data for Sunday, Aug. 11th: High 77°F; Rainfall .00")

(Maximum Ventilation index expected today: 50)



EXTENDED DISCUSSION:



A dry and stable southwesterly flow aloft is forecast on Tuesday, with morning clouds giving way more quickly to sunny skies.  Temperatures should warm into the mid 80s with transport winds turning mostly northerly.



Another mostly sunny and warm day is expected on Wednesday, but an approaching upper-level trough will increase the southwesterly flow aloft, which could lead to very favorable open burning conditions Wednesday afternoon.  A weak cold front is expected to come onshore Wednesday night, with more clouds and cooler temperatures on Thursday.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



Notes:



     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels, and

         winds less than about 15mph.



     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.



     3.  Ventilation Index is the height of the mixing layer (ft) times

         the transport wind speed (mph) divided by 1000.



     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.



This forecast is provided under an agreement between the Oregon Department of

Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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